Hey so I know this is gunna seem pretty noobish of me but i'm new, so try not to downvote me into oblivion.. but say I wanted to use someone's custom tilesets, like those of Luna's or Celianna's. For non-commercial use of course, how would one go about actually getting them into my rpgmaker?

Note that some of the tiles Celianna and I have are not in tileset format. They're either add-ons or parallax tiles. If you want to use those, you'd need an art program (like Photoshop or Gimp) where you can copy/paste object onto a grid.